This invention relates to improved pressure sensitive adhesives, and to compositions and processes for forming such adhesives.
The most common methods of producing pressure sensitive adhesives utilize compositions in which natural or synthetic rubber and a tackifying resin are dissolved in a highly volatile solvent, with the solvent ultimately being evaporated from the composition to leave the dissolved substances in place on a substrate. Such a process has the disadvantage of requiring high thermal energy input for evaporating the solvent, and introducing air pollution and fire hazard problems as a result of emission of and the necessity to control the solvent vapors. In some instances, acrylic resin emulsions have been utilized in lieu of the solvent solutions, with resultant elimination of the fire hazard, but without overcoming the high thermal energy input requirements because of a necessity for evaporating water from the emulsions. Other processes employed in making pressure sensitive adhesives have utilized hot melt compositions, but here again a great deal of energy is required for heating the melt.
More recently, development work has been done with ultraviolet polymerization systems, employing compositions which are liquid at room temperature and are essentially solvent free, and will polymerize to a permanently tacky pressure sensitive film upon exposure to ultraviolet light. Other processes have employed different types of radiation, such as for example electron beam radiation for causing polymerization of the pressure sensitive film. One patent stressing electron beam curing is U.S. Pat. No. 3,897,295, in which the composition subjected to the electron beam includes an acrylate monomer selected from a particular specified group, and a homo polymer or co-polymer of a substance or substances selected from the same group. The polymer is dissolved in the monomer, and the monomer is ultimately polymerized to bind the adhesive film together. This patent states that a thickener such as neoprene may be added to the composition, as may other additives.
In some types of compounds in which acrylate monomers have been employed, such as for example in printing inks and the like, there have been intermixed with the acrylates a photo-initiator material which is adapted to respond to ultraviolet radiation in a manner inducing polymerization of the acrylate to a hardened and cured form.